International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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The Life-Time Load History of a Concrete Structure - Determined by Ultrasound Reflection Technique Otto Kroggel
It was assumed and experimentally proven that ultrasonic waves interact strongly with the interfacial transition zone which provides mechanical bond between the concrete matrix and the aggregates. This interfacial transition zone is of high importance with respect to the load bearing capacity of the heterogeneous material. The starting point of macroscopic cracks can be localised in that area and corresponding to this, the non-linear stress-strain behaviour of concrete must take into account the behaviour of this zone. Results of experimental investigations on 2D-modal concrete by visual observation applying digital image correlation techniques support these assumptions.
Corresponding ultrasound scattering investigations are in good agreement with this model. It was found in loading and unloading experiments, that the transmitted and the backscattered ultrasound energy precisely reflects the distortion process taking place in the interfacial transition zone. The effect of clearly distinguishable elastic and non-elastic effects can be used to get an estimate of the complete loading history especially the maximum load of the structure during its life time.
Because steel in a reinforced concrete structure is mechanically in uncracked zones connected to the surrounding concrete the proposed strategy will also help to assess pre-stressing cables and ordinary reinforcement.
The practical application of the concept needs a reliable database describing the detoriation of the interfacial transition zone depending on the applied maximum load and calibration measurements characterising the undisturbed concrete of the structure under observation. Looking at a bridge as a practical example it is obvious in which part of the structure the maximum loads are acting and where the concrete remained unloaded during the service life. The comparison of these areas with respect to ultrasound attenuation behaviour measured by reflection technique allows an estimate of the loading history.